Inastronomy andspaceflight, alunar orbit (also known as aselenocentric orbit) is anorbit by an object around Earth'sMoon. In general these orbits are not circular. When farthest from the Moon (atapoapsis) a spacecraft is said to be atapolune,apocynthion, oraposelene. When closest to the Moon (atperiapsis) it is said to be atperilune,pericynthion, orperiselene. These derive from names or epithets of themoon goddess.
Lunar orbit insertion (LOI) is anorbit insertion maneuver used to achieve lunar orbit.[1]
Low lunar orbit (LLO) is an orbit below 100 km (62 mi) altitude. These have a period of about 2 hours.[2] They are of particular interest in theexploration of the Moon, but suffer from gravitationalperturbations that make most unstable, and leave only a few orbitaltrajectories possible for indefinitefrozen orbits. These would be useful for long-term stays in LLO.[2]
Most lunar low orbits below 100 km (60 mi) are unstable.[2]
Gravitational anomalies slightly distorting the orbits of some Lunar Orbiters led to the discovery ofmass concentrations (dubbedmascons) beneath the lunar surface caused by large impacting bodies at some remote time in the past.[2][3] These anomalies are large enough to cause a lunar orbit to change significantly over the course of several days. They can cause a plumb bob to hang about a third of a degree off vertical, pointing toward the mascon, and increase the force of gravity by one-half percent.[2]TheApollo 11 first manned landing mission employed the first attempt to correct for the perturbation effect (the frozen orbits were not known at that time). The parking orbit was "circularized" at 66 nautical miles (122 km; 76 mi) by 54 nautical miles (100 km; 62 mi), which was expected to become the nominal circular 60 nautical miles (110 km; 69 mi) when the LM made its returnrendezvous with the CSM. But the effect was overestimated by a factor of two; at rendezvous, the orbit was calculated to be 63.2 nautical miles (117.0 km; 72.7 mi) by 56.8 nautical miles (105.2 km; 65.4 mi).[4]
Study of the mascons' effect on lunar spacecraft led to the discovery in 2001 offrozen orbits occurring at fourorbital inclinations: 27°, 50°, 76°, and 86°, in which a spacecraft can stay in a low orbit indefinitely.[2] TheApollo 15 subsatellitePFS-1 and theApollo 16 subsatellitePFS-2, both small satellites released from the ApolloService Module, contributed to this discovery. PFS-1 ended up in a long-lasting orbit, at 28°inclination, and successfully completed its mission after one and a half years. PFS-2 was placed in a particularly unstable orbital inclination of 11°, and lasted only 35 days in orbit before crashing into the lunar surface.[2]
For lunar orbits with altitudes in the 500 to 20,000 km (300 to 12,000 mi) range, the gravity of Earth leads to orbitperturbations. At altitudes higher than that perturbed two-bodyastrodynamics models are insufficient andthree-body models are required.[5]
Although the Moon'sHill sphere extends to a radius of 60,000 km (37,000 mi),[6] the gravity of Earth intervenes enough to make lunar orbits unstable at a distance of 690 km (430 mi).[7]
TheLagrange points of the Earth-Moon system can provide stable orbits in the lunar vicinity, such ashalo orbits anddistant retrograde orbits.
Some halo orbits remain over particular regions of the lunar surface. These can be used by lunarrelay satellites to communicate with surface stations on thefar side of the Moon. The first to do this was the 2019Queqiao relay satellite. It was placed around Earth-Moon L2 at roughly 65,000 km (40,000 mi) from the Moon.[citation needed]
Since 2022 (CAPSTONE)near-rectilinear halo orbits, using as well a Lagrange point, have been used and are planned to be employed by theLunar Gateway.
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There are three main ways to get to lunar orbit from Earth: direct transfer,low thrust transfer andlow-energy transfer. These take 3–4 days,[word missing] months or 2.5–4 months respectively.[8]
TheSoviet Union sent the first spacecraft to the vicinity of the Moon (or any extraterrestrial object), the robotic vehicleLuna 1, on January 4, 1959.[11] It passed within 6,000 kilometres (3,200 nmi; 3,700 mi) of the Moon's surface, but did not achieve lunar orbit.[11]Luna 3, launched on October 4, 1959, was the first robotic spacecraft to complete acircumlunarfree return trajectory, still not a lunar orbit, but a figure-8 trajectory which swung around thefar side of the Moon and returned to the Earth. This craft provided the first pictures of the far side of the Lunar surface.[11]
Luna 10 became the first spacecraft to actually orbit theMoon and any extraterrestrial body in April 1966.[12] It studiedmicrometeoroid flux, and lunar environment until May 30, 1966.[12] A follow-on mission,Luna 11, was launched on August 24, 1966, and studied lunar gravitational anomalies, radiation and solar wind measurements.
The first United States spacecraft to orbit the Moon wasLunar Orbiter 1 on August 14, 1966.[13] The first orbit was anelliptical orbit, with anapolune of 1,008 nautical miles (1,867 km; 1,160 mi) and aperilune of 102.1 nautical miles (189.1 km; 117.5 mi).[14] Then the orbit was circularized at around 170 nautical miles (310 km; 200 mi) to obtain suitable imagery. Five such spacecraft were launched over a period of thirteen months, all of which successfully mapped the Moon, primarily for the purpose of finding suitableApollo program landing sites.[13]
TheApollo program'sCommand/Service Module (CSM) remained in a lunar parking orbit while theLunar Module (LM) landed. The combined CSM/LM would first enter an elliptical orbit, nominally 170 nautical miles (310 km; 200 mi) by 60 nautical miles (110 km; 69 mi), which was then changed to a circular parking orbit of about 60 nautical miles (110 km; 69 mi). Orbital periods vary according to the sum ofapoapsis andperiapsis, and for the CSM were about two hours. The LM began its landing sequence with a Descent Orbit Insertion (DOI) burn to lower theirperiapsis to about 50,000 feet (15 km; 8.2 nmi), chosen to avoid hittinglunar mountains reaching heights of 20,000 feet (6.1 km; 3.3 nmi). After the second landing mission, the procedure was changed onApollo 14 to save more of the LM fuel for its powered descent, by using the CSM's fuel to perform the DOI burn, and later raising itsperiapsis back to a circular orbit after the LM had made its landing.[15]
Lunar mascons make most low lunar orbits unstable ... As a satellite passes 50 or 60 miles overhead, the mascons pull it forward, back, left, right, or down, the exact direction and magnitude of the tugging depends on the satellite's trajectory. Absent any periodic boosts from onboard rockets to correct the orbit, most satellites released into low lunar orbits (under about 60 miles or 100 km) will eventually crash into the Moon. ... [There are] a number of 'frozen orbits' where a spacecraft can stay in a low lunar orbit indefinitely. They occur at four inclinations: 27°, 50°, 76°, and 86° — the last one being nearly over the lunar poles. The orbit of the relatively long-livedApollo 15 subsatellitePFS-1 had an inclination of 28°, which turned out to be close to the inclination of one of the frozen orbits—but poor PFS-2 was cursed with an inclination of only 11°.
The moon's Hill sphere has a radius of 60,000 kilometres, about one-sixth of the distance between it and Earth.For mean distance and mass data for the bodies (for verification of the foregoing citation), seeWilliams, David R. (20 December 2021)."Moon Fact Sheet".NASA.gov. Greenbelt, MD: NASA Goddard Space Flight Center. Retrieved23 July 2023.
